Remember the early days of PDAs? They revolutionized the concept of a planner by combining calendars, contacts and notes into a compact, easy-to-carry device that could be connected to and synced with your computer.

Fast-forward to today, when you are likely to rely on a smartphone or other handheld device (an iPhone, iPod Touch, BlackBerry, Windows Mobile or Android device, to name a few possibilities) plus multiple computers (desktop at home, laptop at work, netbook on the road, for example) for accessing and managing your personal data. And depending on your job or school, you may find yourself needing to access some of your personal information through a publicly accessible Web service.

While broad and instant access to your e-mail, contacts, meetings and notes can be incredibly advantageous, it presents some challenges -- particularly if you need to add or edit contacts, notes or appointments on the go -- because you need to ensure that all of your sources for information are accurate and current and contain the same set of data.

One way to manage this data syncing is with cloud-based services, where an Internet server acts as a central repository and gateway to your information. A key part of achieving the dream of accurate and reliable information syncing is choosing the right service for your habits, your mobile devices, your particular mix of computers and the applications you use on them.

To help you choose, I've compiled information on six common Internet-based data-syncing options, complete with a summary of their strengths, weaknesses and which platforms and tools they work with. I've covered four free services -- Google, Microsoft's Windows Live, Yahoo Mail and Plaxo -- and two paid services, Apple's MobileMe and hosted Microsoft Exchange.

Now there's no excuse not to have your e-mail, contacts, appointments and notes updated across all of your devices.

Free services

A giant of Web-based services, Google provides a number of online applications and suites, including Gmail, Google Talk, Google Docs and Google Calendar. Gmail and Google Talk also incorporate contacts as part of their combined services.

All of these services can be accessed online from almost any computing or mobile platform (though some devices, such as the iPhone, allow just read-only viewing of some features such as Google Docs), which means that not only does Google function as a sync solution, but it can also serve as a stand-alone Web-based application suite -- or you can use a combination of both approaches.

Be forewarned that some setup is required, and the process is not always intuitive for novice users -- particularly those trying to sync Apple's iCal. Mac users seeking easier integration might want to consider third-party tools for integrating calendars between iCal and Google, such as Spanning Sync ($25/year or $65 for indefinite use) or BusySync ($25, with bulk discounts available).

Thankfully, the Address Book in Mac OS X 10.5.3 and higher offers native sync support for contacts with Gmail -- meaning no additional tools are required.

Google Apps Sync for Microsoft Outlook is a tool that syncs all personal information between Outlook and Google's services. It requires a Google Apps Premier Edition or Education Edition account and is designed for businesses shifting from Microsoft Exchange to Google Apps, not for consumers. A similar tool called Google Apps Migration for Lotus Notes helps businesses shift from Notes to Google Apps.